It has been recognized that the world supply of fossil fuels for the production of energy is being exhausted at ever increasing rates. This realization has resulted in an energy crisis manifested by frequent episodes of fossil fuel shortages, inflation, recession, and war or the threat of war in the fossil fuel producing regions of the world. Moreover, this realization has exacerbated a flight of industry from the colder climates to warm climates, resulting in severe economic and social stress and dislocations. The recurring energy crises threaten peace, stability, and economic growth and development.
A solution to the energy crisis lies in harvesting energy from heretofore underutilized energy sources and in more effective use of energy from existing energy sources. To that end, the present invention deals with energy harvesting, as well as with energy conservation, pollution control, and economic growth. It does this by the development of new hydrogen storage materials particularly suited for use in heat pumps.
Conventional heat pumps are of the compressor type. The compressor is noisy, consumes large amounts of electrical power, requires frequent maintenance, is a source of vibration, and use environmentally undesirable fluorocarbon refrigerants. Because of these numerous drawbacks, much research has gone into the development of chemical heat pumps, particularly hydride heat pumps. See, for example, U.S. Pat. No. 4,044,819, issued to Cottingham, U.S. Pat. No. 4,200,144 issued to Sirovich, and "Metal Hydride Heat Pump", D. A. Rohy, T. A. Argabright and G. D. Wade, Proc, 17th Inter Soc Energy Conv Eng Conf (1982) for descriptions of typical hydride heat pump systems.
These hydride heat pumps operate on the principle of reversible storage of hydrogen. This is explained as shown below: EQU M+1/2H.sub.2 =MH+Q,
where M is a hydride former. M reacts with hydrogen to form a hydride MH. This process is exothermic and it releases a quantity of heat, Q, which can be usefully utilized. On the other hand, the reverse reaction involves supplying a quantity of heat, Q, to the hydride, MH, to regenerate the original hydride former M, releasing hydrogen and producing a cooling effect. This process of producing heating and/or cooling effect involves only the transfer of hydrogen, thus eliminating the need for a compressor. As a result the problems of the compressor type heat pump, i.e., the noise, vibration, and other problems are reduced or eliminated.
In a typical hydride heat pump system, two different metal alloy hydrides are paired. The hydrides are contained in beds which are thermally and physically isolated, but interconnected to permit the flow of hydrogen. This is represented schematically by FIG. 1. Initially, hydride A is substantially hydrided and hydride B is substantially unhydrided.
A typical heat pump cooling cycle consists of four steps, which are illustrated in FIG. 2. As shown in the FIGURE:
Step 1--Isothermal Hydrogen Desorption at A and Absorption at B
Bed A receives the heat Q.sub.CA at T.sub.C. Bed B absorbs hydrogen (released from A) and deilvers the heat -Q.sub.MB to the surroundings at T.sub.M.
Step 2--Sensible Heating
Bed A is heated to T.sub.M from T.sub.C, absorbing heat Q.sub.SA. Bed B is heated to T.sub.H from T.sub.M, absorbing heat Q'.sub.SB.
Step 3--Isothermal Hydrogen Desorption at B and Absorption at A
Bed B receives heat (Q.sub.HB) from a heat source at T.sub.H. Bed A absorbs hydrogen released from B and delivers the heat -Q.sub.MA to a heat sink at T.sub.M.
Step 4--Sensible Cooling
Bed A releases the heat -Q'.sub.SA and is cooled from T.sub.M to T.sub.C, back to its original state. Bed B releases the heat -Q.sub.SB and is also cooled from T.sub.H to T.sub.M, back to its original state.
Different hydrides will display differing pressure versus composition (H/M) profiles. At a given temperature, each hydride will have a characteristic pressure at which it will begin to absorb significant amounts of hydrogen. The pressure will remain relatively constant until a characteristic quantity of hydrogen is absorbed. At that point, exposing the hydride to more hydrogen will not cause a significant composition change. In order for hydrogen to flow from the bed containing hydride A to the bed containing hydride B during Step 1, the hydride pair must display properties such that there is an appropriate pressure differential between A and B at temperatures T.sub.C and T.sub.M, as shown above. Likewise, in order for hydrogen to flow from bed B to bed A during Step 3, the pair of hydrides must have properties such that there is a sufficient pressure differential between B and A at temperatures T.sub.H and T.sub.M respectively.
See "A Thermodynamic Analysis of a Metal Hydride Heat Pump", A. Abelson and J. S. Horowitz, Argonne National Laboratory, No. 809427, for a fuller discussion of the thermodynamic considerations.
By providing pairs of matched hydrides operating in alternating sequence, it is possible to provide continuous cooling to a load. Also, by exploiting the exothermic reaction of hydrogen absorption, it is also possible to employ the hydride heat pump as a heating device.
In addition the hydride heat pump can use any source of heat; i.e., gas, electricity, solar, waste heat, ground water, etc, thus providing for more efficient and flexible energy utilization. Due to the modular, solid state nature of the hydride system and because hydrogen gas is the only working medium, these systems also allow packaging flexibility.
Solid hydrides suitable for the reversible storage of hydrogen in heat pump applications must have several characteristics, such as: the useful capacity must be large at a given operating temperature; they must have the ability to operate over a broad range of temperatures; they must have good hydrogen absorption/desorption kinetics; the materials must be structurally stable to ensure long cycle life; the hysterisis between hydrogen absorption and desorption pressures of the materials should be small; the thermal conductivity of the materials should be reasonably high; and the cost should be reasonably low. If the materials fail to possess any one of these characteristics, they will not find acceptance for wide scale commercial utilization.
A relatively low desorption temperature is necessary for efficient utilization of the available exhaust heat from vehicles, machinery, or other similar equipment.
Reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Favorable kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time.
While the design of the hydride heat pump system in respect to apparatus has developed some sophistication, hydride materials development has lagged. No hydride material found in the prior art meets all the requirements necessary for commercial exploitation in heat pump systems.
In the field of hydrogen storage materials, the most commonly prepared systems are based on single phase crystalline bi-metal or tri-metal hydride alloys. Such prior art crystalline materials, however, have not been able to meet even the minimum requirements necessary for wide scale commercial acceptance. A basic limitation of many of the prior art crystalline materials have been their low hydrogen storage capacity relative to the weight of the material.
Another limitation is that many of these materials possess desorption temperatures which are unreasonably high for many applications.
Many of the prior art crystalline materials also are quite susceptible to poisoning by exposure to contaminants in the hydrogen gas or from the ambient environment. For example, many crystalline materials can be poisoned by the presence of oxygen at levels as low as parts per million concentrations. Once contaminated, the storage characteristics of the materials degrade significantly, thereby rendering these materials unacceptable for use without reactivating the materials, which is expensive and complicated.
The density of hydrogen storage sites or interstitial sites is also limited due to specific stoichiometries in the single phase crystalline host structures. In single phase crystalline host materials, the catalytically active sites are relatively limited in number and result from accidentally occurring surface irregularities which interrupt the periodicity of the single phase crystalline structure. A few examples of such surface irregularities are dislocation sites, crystal steps, surface impurities, and foreign absorbates. These irregularities typically occur only in relatively few numbers on the surface of a single phase crystalline material and not throughout its bulk.
The density of catalytically active sites can be increased to a limited extent by mechanical cracking of the single phase crystalline structure or by forming a powder therefrom to increase the surface area. The powders can present a utilization problem, because when the stored hydrogen is later released from the hydride, the powder particles or fractioned segments thereof may be in and carried off by the hydrogen gas as the gas is pumped to its point of utilization as a fuel.
The prior art metallic host single phase crystalline hydrogen storage materials include magnesium, magnesium-nickel, vanadium, iron-titanium, lanthanum pentanickel and others. No such prior art material, however, has all of the required properties i.e., useful storage capacity at operational temperatures, acceptable absorption/desorption kinetics, etc. required for a hydride heat pump medium with widespread commercial utilizability. For example, a crystalline magnesium hydride is theoretically capable of storing hydrogen at approximately 7.6% by weight computed using the formula: EQU percent storage=H/(H+M),
where H is the weight of the hydrogen stored and M is the weight of the storage material (all storage percentages hereinafter referred to are computed based on this formula). However, magnesium's other hydrogen storage characteristics make it commercially unaccceptable for widespread use as a heat pump material.
Magnesium is very difficult to activate. For example, U.S. Pat. No. 3,479,165 discloses that it is necessary to activate magnesium to eliminate surface barriers. This activation must occur at temperatures of 400.degree. C. to 425.degree. C. and at a pressure of 1000 psi for a period of several days in order to obtain a reasonable (90%) conversion to the hydride state. Furthermore, desorption of such hydrides typically requires heating to relatively high temperatures before hydrogen desorption begins. The aforementioned patent states that the MgH.sub.2 material must be heated to a temperature of 277.degree. C. before desorption initiates. Even significantly higher temperatures and times are required to reach an acceptable operating output. The high desorption temperature makes the magnesium hydride unsuitable for many applications, in particular applications wherein it is desired to utilize waste heat for desorption, such as exhaust heat from combustion engines. The high temperature and high pressure required in the charging process also limit the use of magnesium in many applications.
The other aforementioned single phase crystalline materials also have not achieved commercial acceptance. For example, Mg.sub.2 NiH.sub.4 does not have matching pairs with proper pressure composition profiles to extract enough hydrogen at operational conditions to be useful in a heat pump. VH.sub.2 and LaNi.sub.5 H.sub.6 are too expensive for commercial use. Another disadvantage in some of the prior art materials has been the lack of ability to charge the materials in an acceptable amount of time. In summary, none of the prior art hydrogen storage materials have all the desired properties necessary for commercial acceptability.
One alloy system that has been utilized as a hydrogen storer is that containing various combinations of titanium, vanadium, manganese and iron. See, for example, U.S. Pat. No. Re. 30,083 in the name of Reilly et al, which is a reissue of U.S. Pat. No. 3,922,872, U.S. Pat. No. 4,358,316 in the name of Liu et al, U.S. Pat. No. 4,457,891 in the name of Bernauer et al, and U.S. Pat. No. 4,488,906 in the name of Gondo et al. All of these references teach the use of an alloy with a major phase of either titanium or manganese, and little or no vanadium. U.S. Pat. No. 4,446,101 in the name of Bernauer et al does disclose a material with a higher fraction of vanadium. However, no dislcosure is made as to the material's structure or pressure versus (H/M) behavior.
While it has been postulated that a particular crystalline structure is required for hydrogen storage, see, for example, "Hydrogen Storage in Metal Hydride", Scientific American, Vol. 242, No. 2, pp. 118-129, February, 1980, it is possible to overcome many of the disadvantages of the prior art materials by utilizing a different class of materials, disordered hydrogen materials. For example, U.S. Pat. No. 4,265,720 to Guenter Winstel for "Storage Materials for Hydrogen" describes a hydrogen storage body of amorphous or finely crystalline silicon. The silicon is preferably a thin film in combination with a suitable catalyst and on a substrate.
Laid-open Japanese Patent Application No. 55-167401, "Hydrogen Storage Material," in the name of Matsumato et al, discloses bi or tri element hydrogen storage materials of at least 50 volume percent amorphous structure. The first element is chosen from the group Ca, Mg, Ti, Zr, Hf, V, Nb, Ta, Y and lanthanides, and the second from the group Al, Cr, Fe, Co, Ni, Cu, Mn and Si. A third element from the group B, C, P and Ge can optionally be present. According to the teaching of No. 55-167401, the amorphous structure is needed to overcome the problem of the unfavorably high desorption temperature characteristic of most crystalline systems. A high desorption temperature (above, for example, 150.degree. C.) severely limits the uses to which the system may be put. For example, a heat pump used as an air conditioner would have to perform at ambient temperatures. The materials suggested by Matsumoto et al do not have suitable matching pairs with appropriate pressure versus composition curves.
According to Matsumoto et al, the material of at least 50% amorphous structure will be able to desorb at least some hydrogen at relatively low temperatures because the bonding energies of the individual atoms are not uniform, as is the case with crystalline material, but are distributed over a wide range. Such an amorphous material will not have the flat hysteresis curve (pressure isotherm) characteristic of crystalline materials. Hence, at least some hydrogen will be desorbed at a relatively low temperature.
Matsumoto et al claims a material of at least 50% amorphous structure. While Matsumoto et al does not provide any further teaching about the meaning of the term "amorphous," the scientifically accepted definition of the term encompasses a maximum short range order of about 20 Angstroms or less.
The use by Matsumato et al of amorphous structure materials to achieve better desorption kinetics due to the non-flat hysteresis curve is an inadequate and partial solution. The other problems found in crystalline hydrogen storage materials, particularly low useful hydrogen storage capacity at ambient temperature, remain.
However, even better hydrogen storage results, i.e., long cycle life, good physical strength, low absorption/desorption temperatures and pressures, reversibility, and resistance to chemical poisoning, may be realized if full advantage is taken of modification of disordered metastable hydrogen storage materials. Modification of disordered structurally metastable hydrogen storage materials is described in the commonly assigned U.S. Pat. No. 4,431,561 of Stanford R. Ovshinsky, Krishna Sapru, Krystyna Dec, and Kuochih Hong, for "Hydrogen Storage Materials and Method of Making the Same". As described therein, disordered hydrogen storage materials, characterized by a chemically modified, thermodynamically metastable structure, can be tailor-made to possess all the hydrogen storage characteristics desirable for a wide range of commercial applications. The modified hydrogen storage material can be made to have greater hydrogen storage capacity than do the single phase crystalline host materials. The bonding strengths between the hydrogen and the storage sites in these modified materials can be tailored to provide a spectrum of bonding possibilities thereby to obtain desired absorption and desorption characteristics. Disordered hydrogen storage materials having a chemically modified, thermodynamically metastable structure also have a greatly increased density of catalytically active sites for improved hydrogen storage kinetics and increased resistance to poisoning.
The synergistic combination of selected modifiers incorporated in a selected host matrix provides a degree and quality of structural and chemical modification that stabilizes chemical, physical, and electronic structures and conformations amenable to hydrogen storage.
The framework for the modified hydrogen storage materials is a lightweight host matrix. The host matrix is structurally modified with selected modifier elements to provide a disordered material with local chemical environments which result in the required hydrogen storage properties.
Another advantage of the host matrix described by Ovshinsky, et al. is that it can be modified in a substantially continuous range of varying percentages of modifier elements. This ability allows the host matrix to be manipulated by modifiers to tailor-make or engineer hydrogen storage materials with characteristics suitable for particular applications. This is in contrast to multicomponent single phase host crystalline materials which generally have a very limited range of stoichiometry available. A continuous range of control of chemical and structural modification of the thermodynamics and kinetics of such crystalline materials therefore is not possible.
A still further advantage of these disordered hydrogen storage materials is that they are much more resistant to poisoning. As stated before, these materials have a much greater density of catalytically active sites. Thus, a certain number of such sites can be sacrificed to the effects of poisonous species, while the large number of unpoisoned active sites still remain to continue to provide the desired hydrogen storage kinetics.
Another advantage of these disordered materials is that they can be designed to be mechanically more flexible than single phase crystalline materials. The disordered materials are thus capable of more distortion during expansion and contraction allowing for greater mechanical stability during the absorption and desorption cycles.